When a player in any game chooses (on purpose) to not play their best. Normally this is because they are too superior, they want to hustle you, or they are too lazy to play their best with nothing on the line.

To put someone on the spot in front of a large crowd, often resulting in having the target's dignity drop like a sandbag.

Rebel: That is impossible, its diameter is smaller than 2 meters!
Luke: It is very easy, I shoot whomp rats back home and they are no bigger than 2 meters.
Rebel: Dude, why did you sand bag me back there?

The act of embarrassing someone by making them look foolish or stupid in front of a large group of people.

Usually it is unintentional or by accident. it can either be a dick move or the person is completely overreacting

first coined in the robot chicken star wars

BJ- (to a group of females) Yeah i'm the vocal coach for the band. I taught them everything they know.
Jay- no you're not dude, aren't you a janitor?
BJ- why did you have to sandbag me in front of those girls man? i was gonna get some!
Jay-what are you talking about? what does "sandbag" even mean? they were out of your league anyway
BJ- You're a total sandbagger man...

(verb) To screw around or work on other things while you are supposed to be completing a task or working on something.

Bob looked to be a promising hire with his Princeton degree and impressive summer internships. But when his boss checked his internet use log, it was obvious Bob had been sandbagging up a storm: posting daily from work on his "Notorious B.O.B. in da hizzouse" blog and spending an inordinate amount of time on midgetporn.com.

To shoot-down or put-down someone hardcore. Can be involuntary; to point out ones flaws
Sandbagged, Sandbagging.

-Ronnie: "Don't you like this picture I drew? I'm quite proud of it."
-Dylan: "No, actually Ronnie, that's a terrible picture. You colored outside the lines completely and you have no artistic talent."
-Bubz: "OHHH OHHH! SANDBAGGED! RONNIE, YOU JUST GOT SANDBAGGED BY DYLAN THE SANDBAG KING!"